The Endocrine System: Overview

• The endocrine system is the second controlling system of the body
• Chemical messages are released into the blood stream
• Endocrine system is slower compared to the nervous system

Hormone Control

• Hormones control processes such as reproduction, growth, development, mobilization of body defenses, maintenance of homeostasis, and regulation of metabolism
• Hormones are produced by cells of various endocrine organs
• Hormones are transported by blood to target cells, regulating their activity

Endocrine Organs

• Include the hypothalamus, pineal gland, pituitary gland, thyroid gland, thymus gland, adrenal glands, pancreas, testes (male), and ovaries (female)

Pineal Gland

• Found in the epithalamus of the diencephalon in the brain
• Melatonin is secreted
• Melatonin is to establish wake and sleep cycles
• High melatonin levels at night induce drowsiness, whereas low levels during the day promote wakefulness

Pituitary Gland

• Attached to the hypothalamus by a stalk
• Has two functional lobes: the anterior and posterior pituitary

Anterior Pituitary Hormones

• This produces six hormones

Growth Hormone (GH)

• Metabolic hormone
• Effects growth of skeletal muscles and long bones
• Amino acids are built into proteins, and fats are broken down for energy

Prolactin (PRL)

• This stimulates milk production and aids in maintenance after childbirth
• The function in males is unknown under normal circumstances

Adrenocorticotropic Hormone (ACTH)

• Regulates activity in the cortex of the adrenal gland

Thyrotropic Hormone (TH)

• Influences the growth and activity of the thyroid gland, also known as thyroid-stimulating hormone (TSH)

Gonadotropic Hormones

• Regulate hormonal activity of the gonads

Follicle-Stimulating Hormone (FSH)

• In females, this stimulates follicle development in the ovaries, which helps with egg development
• In males, this stimulates sperm development in the testes

Luteinizing Hormone (LH)

• In females, this triggers the ovulation process
• In males, this stimulates the production of testosterone in the testes

Hypothalamus

• Located in the diencephalon of the brain
• Produces releasing and inhibiting hormones
• These regulate the anterior pituitary hormone release/inhibition
• Hormones are transported to the posterior pituitary for storage and later release

Posterior Pituitary Hormones

• Stores oxytocin and antidiuretic hormone (ADH)

Oxytocin

• This stimulates contractions of the uterus during labor and can also help restore the uterus size after labor
• It causes milk ejection during breastfeeding or when babies cry
• It acts as a bonding hormone in both males and females

Antidiuretic Hormone (ADH)

• This inhibits urine production and causes vasoconstriction, leading to increased blood pressure

Thyroid Gland

• Located at the base of the throat
• It consists of two lobes connected by an isthmus
• Produces thyroid hormone and calcitonin

Thyroid Hormone

• Metabolic hormone
• Controls the rate of glucose conversion to ATP
• Targets all living cells in the body
• It has two forms that both contain iodine
  • Thyroxine (T4)
  • Triiodothyronine (T3)
• A lack of iodine causes a goiter, which causes enlargement of the thyroid gland
• Hypothyroidism means too little thyroid hormone
• Hyperthyroidism means too much thyroid hormone

Calcitonin

• This decreases blood calcium levels by depositing calcium on bone

Parathyroid Glands

• Tiny masses on the posterior of the thyroid gland, with two on each lobe
• Parathyroid hormone (PTH) is secreted
• Works opposite to calcitonin

Parathyroid Hormone (PTH)

• This increases calcium levels in blood
• Special cells called osteoclasts are stimulated to remove calcium from bone
• Kidneys and intestines are stimulated to absorb more calcium

Thymus Gland

• Located posterior to the sternum
• Produces thymosin
• Large in infants/children and small in adults
• Important for developing the immune system (white blood cells)

Adrenal Glands

• Located on top of the kidneys
• Two divisions with different functions: the cortex and the medulla
• Hormones are produced in both divisions

Adrenal Cortex Hormones

• Mineralocorticoids, Glucocorticoids, and Sex hormones

Mineralocorticoids

• The target organ is the kidney
• Regulates mineral, water, and electrolyte content in blood
• The main hormone is Aldosterone

Glucocorticoids

• These are released in response to ACTH
• Cortisone and cortisol are included
• The purpose is to promote cell metabolism and to help resist long-term stress

Sex Hormones

• Androgens secreted in males
• Estrogen is secreted in females

Adrenal Medulla Hormones

• Two Catecholamines are produced
  • Epinephrine (adrenaline)
  • Norepinephrine (noradrenaline)
• Prepares the body to deal with short-term stress, "fight or flight"
• Increase heart rate and blood pressure
• Blood glucose levels are increased
• Dilates passageways to lungs

Pancreas

• A mixed gland that produces hormones and enzymes
• Pancreatic islets produce hormones that are antagonists for maintaining blood sugar homeostasis
• Insulin and glucagon
• Insulin is stimulated when there is high blood glucose, which allows glucose to cross over plasma membranes into the cells
• Glucagon is stimulated when there is low blood glucose, which communicates with the liver to release glucose into the blood

Ovaries

• Produce estrogens and progesterone

Estrogens

• Stimulate development of secondary sex characteristics – hips, breasts, etc
• Matures reproductive organs, helps control the menstrual cycle and maintain pregnancy, and prepares breasts to produce milk

Progesterone

• The menstrual cycle is brought about by acting with estrogen
• Helps in implantation of an embryo in the uterus

Testes

• This produces a group of hormones called androgens, which are important to testosterone

Testosterone

• This stimulates the development of secondary sex characteristics: deepening of voice, growth of muscle, growth (height), etc
• Matures reproductive organs, stimulates male sex drive, and is required for sperm production

Placenta

• A temporary organ in a pregnant uterus
• This produces estrogen and progesterone
• Produces human chorionic gonadotropin (HCG)

Glands

• Two types of glands within the body
  • Exocrine
  • Endocrine

Exocrine Glands

• Have ducts
• Products are secreted onto the body surface or into body cavities, e.g., sweat glands, salivary glands, and sebaceous glands

Endocrine Glands

• Ductless and secrete hormones directly into the blood
• Both the pancreas and gonads are endocrine glands

Hormone Classifications

Water-Soluble Hormones

• Composed of amino acids and are derived from proteins
  • Hypothalamic hormones
  • Pituitary hormones
  • Catecholamines
  • Pancreatic hormones

Fat-Soluble Hormones

• Steroid hormones made from cholesterol
  • Cortisol
  • Aldosterone
  • Androgens
  • Thyroid hormone is an exception because it is made from proteins

Hormone Release Control

• Negative feedback mechanism
• Stimuli (more than one type)
• Negative feedback controls hormone levels in blood
• Low hormone levels trigger a release of more hormone
• Rising hormone levels in blood stop the release of hormones

Stimuli Types

• Three types of stimuli communicate to an endocrine organ to secrete a hormone
  • Hormonal stimuli
  • Humoral stimuli
  • Neural stimuli

Hormonal Stimuli

• Endocrine glands are activated by hormones secreted from other endocrine glands

Humoral Stimuli

• Changing blood levels of certain ions stimulate hormone release

Neural Stimuli

• Nerve impulses simulate hormone release
• The sympathetic nervous system controls hormone release

Hormone Communication

• Hormones affect specific cells or organs ("target cells" or "target organs")
• Hormones identify cells or organs through receptor proteins

Hormone Action

• Two ways hormones trigger changes in their target cells
  • A second-messenger system
  • Direct gene activation

Second Messenger System

• A hormone binds to a receptor protein on the membrane
• Activates an enzyme through a series of reactions
• Enzyme then produces a second messenger molecule
• Allows second messenger molecule to directs changes in the cell
• Utilized by water-soluble hormones

Direct Gene Activation

• A hormone enters the nucleus after circulating through the target cell membrane
• Binds to a receptor protein to form a hormone-receptor complex
• Allows hormone-receptor complex to bind to the cell's DNA
• Activates protein synthesis on specific protein genes, which allows it to make new proteins
• Utilized by fat-soluble hormones

Effects of Hormones

• Possible effects of hormones upon arrival at target site:
  • Changes in plasma membrane permeability or electrical state
  • Turns on or off transcription (to make proteins or stop making proteins)
  • Activates or inactivates enzymes
  • Stimulates or inhibits cell division
  • Promotes or inhibits secretion of a product
• The same hormone may produce varied effects depending on the specific target cell involved